System for distributing locally generated energy to multiple load units

ABSTRACT

A system for distributing locally generated energy from at least one renewable DC source to a plurality of local load units of the system, including, for each load unit: an input terminal configured to connect to a grid, and an output terminal configured to connect to at least one load. Further for each load the system includes an inverter including an inverter input and an inverter output, wherein the inverter input is connected to the at least one renewable DC source and the inverter output is connected to the input terminal and to the output terminal of the respective load unit, and wherein the inverter is configured to convert a direct current at the inverter input into an alternating current at the inverter output. The system also includes a power meter including a power meter input connected to the input terminal of the respective load unit, wherein the power meter is configured to determine a current power consumption from the grid, and wherein the power meter includes a power meter output connected to the inverter of the respective load unit, and wherein the power meter is configured to transmit data relating to the current power consumption from the grid to the inverter. The inverter of the respective load unit is configured to determine an input DC voltage applied to its inverter input and to determine a power to be currently converted from the applied input DC voltage and the current power consumption data transmitted thereto.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Applicationnumber PCT/EP2018/056218, filed on Mar. 13, 2018, which claims priorityto German Patent Application number 10 2017 108 121.6, filed on Apr. 13,2017, and is hereby incorporated by reference in its entirety.

FIELD

The disclosure relates to a system for the improved distribution oflocally generated energy. The system has a plurality of local loadunits, and the locally generated energy can be provided by means of ashared renewable DC source. Furthermore, the disclosure also relates toa retrofit kit for an electrical distribution panel.

BACKGROUND

The problem of distributing locally generated energy arises, forexample, in apartment buildings if a plurality of families or usergroups wish to use a common local energy source but are also connectedto a grid, for example a public energy grid. The distribution of theenergy flows to the user groups, the capture of the consumed amounts ofenergy in each case and administrative provisions of the grid operatorscause the sharing of decentralized, in particular renewable, energysources to be complicated and consequently cost-intensive. Therefore,either local energy generation installations for only one user grouphave previously been operated or large installations have fed into alarger grid in order to be distributed and captured using the alreadyexisting grids and energy meters. Since the feed in of regenerativeenergy to the grid is financially less and less attractive in manycountries, the self consumption from local renewable energy sources isbecoming more interesting. However, this is also useful at the same timesince the grid are therefore not loaded with the fluctuating amount ofregenerative energy and local generation and local consumption thereforecome together.

The sharing of a local regenerative energy source has only been rarelyimplemented even though the self consumption rate can be considerablyincreased by sharing a photovoltaic installation on the roof of anapartment building, for example. With the increasing number of differentusers, the variation in the energy consumption habits increases, andload peaks and troughs in the grid are statistically compensated by thelarge number of consumers. In a similar manner, the distributed use byindividuals being active during the day or at night or families withdifferent habits in terms of cooking, washing or using other electricalor electronic devices can homogenize the energy consumption in anapartment building. Therefore the locally energy generation installationcan be used in an optimized manner if the energy is distributed todifferent user groups. If there is nevertheless still surplus energy, anenergy storage can be provided. However, this energy storage can beconsiderably smaller and therefore more cost-effective than it would bethe case in one individually used installation or indeed a plurality ofindividually used installations.

The prior art discloses the practice of providing a central controldevice for sharing a local regenerative energy source by a plurality ofuser groups, as disclosed in EP 2 993 752 A2, for example. In this case,an inverter and an electricity meter are provided for each user groupand each inverter is controllably connected to a central control device,wherein the central control device divides the electrical power to beprovided among the user groups. The installation, operation andmaintenance of such a central control device are associated with effortand costs.

SUMMARY

In the present disclosure the electrical power to be provided can bedistributed among the user groups in a simple and cost-effective mannereven without a central control device.

A system according to the disclosure for distributing locally generatedenergy from at least one renewable DC source to a plurality of localload units of the system comprises, for each load unit, an inputconfigured to be connected to an grid and an output configured to beconnected to at least one load. The load units each comprise an inverterwith an inverter input and an inverter output, wherein the inverterinput is connected to the DC source and the inverter output is connectedto the input and to the output of the respective load unit. The inverterconverts a direct current at the inverter input into an alternatingcurrent at the inverter output. The load units also comprise a powermeter with a power meter input connected to the input of the respectiveload unit in order to determine a current power consumption from thegrid, and with a power meter output connected to the inverter of therespective load unit. The power meter output transmits data relating tothe current power consumption from the grid to the inverter. At the sametime, the inverter of the respective load unit determines an input DCvoltage applied to its inverter input and determines a power to becurrently converted from the applied input DC voltage and thetransmitted power consumption data.

In this manner, the load units are arranged parallel to one anotherbetween the grid and the local DC source, and the associated invertersact autonomously without a superordinate controller. The power meter maybe an electricity meter, as is usually present in the associated loadunit, for example an apartment in an apartment building. The power meterdetermines the current power consumption from the grid and transmitsthis value to the associated inverter in a wired or wireless manner.Loads can be connected to the output of the load unit. These may be, forexample, loads which are conventional in a household such as cookers,dishwashers, washing machines, lighting means or the like. If theinverter is active, that is to say it converts direct current present onthe input side into grid-compliant alternating current, the loads can besupplied with power from the grid and from the local DC source. In thiscase, single-phase or multi-phase inverters can be used. Since theelectrical loads are connected in many countries in a manner dividedamong a plurality of phases of a grid, inverters which are adapted tothe respective conditions will be selected. The associated inverter canuse the transmitted power meter data to determine the power which wouldadditionally need to be converted from the local DC source at thecurrent time in order to completely supply the loads from the local DCsource. At the same time, the associated inverter also determinesmeasured values which characterize the input DC voltage applied to itsinput, for example the amplitude of the input DC voltage. This may alsobe a signal which is modulated onto the input DC voltage and, like theinput DC voltage itself, is generated by the local DC source.

In many generators, the terminal voltage falls during loading; and alocal DC source provided as a photovoltaic generator can behave in asimilar manner, for example, or this generator behavior can be simulatedby a DC chopper stage (DC/DC converter) connected to the photovoltaicgenerator. The terminal voltage can therefore act as a measure of theloading of the generator and power reserves of the generator can beinferred from the level of the terminal voltage. A DC chopper stageconnected to the photovoltaic generator could also modulate a signalfrom which the power reserves of the photovoltaic generator can bederived. If the inverter associated with the respective load unit nowdetermines a current input voltage level or a corresponding signal whichvaries with increasing loading, the inverter can determine whether thelocal DC source still has power reserves, for example by means of acomparison with stored threshold values, characteristic curves or thelike. A power to be currently converted can be determined from thecomparison of indicators of power reserves and the current powerconsumption from the grid. This may be, for example, the power whichwould need to be additionally converted from the local DC source at thecurrent time in order to completely supply the loads from the local DCsource if there are sufficient power reserves. Alternatively, the powerto be currently converted may correspond to a portion of the power whichwould need to be additionally converted from the local DC source at thecurrent time in order to completely supply the loads from the local DCsource.

In one advantageous embodiment of the system according to thedisclosure, the inverter of the respective load unit determines from theinput DC voltage applied to its inverter input a maximum possible powerwhich can be currently converted according to a predefined firstcharacteristic curve. Such a characteristic curve may establish, forexample, a linear relationship between the input DC voltage and themaximum possible power which can be converted by the relevant inverter,with the result that, in the case of average loading of the DC source,the power converted by the inverter can be adapted to the power requiredby the connected loads. The inverter could therefore increase itscurrently converted power along the characteristic curve, for example,until all loads have been supplied or the maximum performance of the DCsource, that is to say the maximum possible power which can be currentlyconverted, has been reached.

If the maximum possible power which can be currently converted isgreater than or equal to the power consumption from the grid, anotheradvantageous embodiment of the system according to the disclosureprovides for the inverter of the respective load unit to determine thepower to be currently converted in such a manner that the powerconsumption from the grid tends toward a preset limit value. In acontinuous method, for example, the inverter of the respective load unitdetermines the level of the current power consumption from the grid and,in one embodiment of the present disclosure, attempts to adapt the powerconverted by it in such a manner that it corresponds to storedspecifications. For example, an upper limit for the power consumptionfrom the grid by the load unit under consideration could be set, withthe result that the associated inverter increases or reduces the powerconverted by it until this upper limit has been reached.

Another variant could involve storing a limit value for the powerconsumption from the local DC source, with the result that the inverterincreases its currently converted power only until this limit value hasbeen reached, and any further consumption of the connected loads issupplied from the grid. It is also conceivable, in one embodiment, forthe control objective for the inverter of the respective load unit to bethat a particular portion of the energy consumed in the respective loadunit is always taken from the local DC source. The prerequisite for thisis that the local DC source has sufficient power reserves and there is aneed from the connected loads.

In one embodiment of the system according to the disclosure, the presetlimit value is 0 kW, which means that, if the maximum possible powerwhich can be currently converted is at least equal to the powerconsumption from the grid, the inverter of the respective load unitdetermines the power to be currently additionally converted in such amanner that it corresponds to the level of the power consumption fromthe grid. The inverter of the respective load unit consequentlyincreases its currently converted power by the level of the previouslydetermined power consumption from the grid, with the result that theloads connected to the respective load unit are then completely suppliedfrom the local DC source.

If the maximum possible power which can be currently converted is lessthan the power consumption from the grid, the inverter of the respectiveload unit determines the power to be currently converted in anotherembodiment of the system according to the disclosure in such a mannerthat the power to be currently converted corresponds to the maximumpossible power which can be currently converted. This minimizes thepower consumption from the grid. This means that, if the power offeredby the local DC source is not sufficient to supply all loads connectedto the respective load unit to the desired extent, for example, theinverter of the respective load unit increases the power converted byit, at least by the maximum possible amount.

In one advantageous embodiment of the system according to thedisclosure, the inverters of the respective load units are provided asunidirectional and possibly DC-isolating inverters. Even if anelectrical storage is provided in a system according to the disclosureon the DC voltage side, a flow of energy from the AC voltage side to theDC voltage side of the inverters of the system according to thedisclosure is not intended. Therefore, inverters whose semiconductorswitches are optimized for the direction of energy flow from DC to ACare sufficient and therefore reduce the costs of the system according tothe disclosure. In order to ensure electrical decoupling of the DCvoltage side from the AC voltage side, the inverters may be provided asDC-isolating inverters.

In another embodiment of the system according to the disclosure, thefirst characteristic curves of the respective load units differ from oneanother. This makes it possible to stipulate different controlspecifications for the inverters of the respective load units. Thedifferent characteristic curves may be stored in control components ofthe respective inverters. Even though the same input DC voltage isapplied to all inverters of the system according to the disclosure, theinverters can determine different maximum possible powers which can becurrently converted for the respective load unit from the input DCvoltage using different characteristic curves. This feature also makesit possible to distribute the locally generated energy in an unevenmanner, if this is desired, without a superordinate controller. However,it goes without saying that, if uniform distribution is desired, thesame characteristic curve can also be stored in each inverter of thesystem according to the disclosure, with the result that the same powerto be currently converted is determined by the inverters in the case ofthe same energy consumption of the connected loads.

In another embodiment of the system according to the disclosure, for atleast one load unit, the power converted by the inverter of the relevantload unit is summed within a time window. If a threshold value ispredefined for the summed power—therefore an energy threshold value—ofthe relevant load unit, the inverter of the relevant load unit reducesits currently converted power to zero upon reaching the threshold value.If different time windows are also provided for different load units ina manner distributed over the day, the energy provided by the local DCsource can thus be divided into portions as it were and can be dividedby the inverters of the system according to the disclosure among thedifferent load units in a manner distributed over the day without theneed for a superordinate controller.

In one embodiment, the system according to the disclosure also comprisesa storage unit. The storage unit comprises at least one battery and abidirectional DC chopper, wherein a first terminal of the bidirectionalDC chopper is connected to the DC source and a second terminal of thebidirectional DC chopper is connected to the battery. The bidirectionalDC chopper determines a maximum possible power which can be currentlystored from an input DC voltage applied to its first terminal accordingto a predefined second characteristic curve. In a similar manner to theinverters in the load units, the bidirectional DC chopper in the storageunit determines whether the local DC source still has power reservesfrom the current input voltage level, for example by means of acomparison with stored threshold values, characteristic curves or thelike. If previously unused power is still locally available, it can bestored in the battery. In another embodiment, the predefined secondcharacteristic curve differs from the predefined first characteristiccurve. This makes it possible to achieve the situation, for example, inwhich the loads connected to the load units are first of all suppliedwith electrical energy and the battery is charged only secondarily. Ifthe system according to the disclosure is used to optimize selfconsumption, direct local consumption will be given preference over thestorage of energy. On the one hand, this makes it possible to avoidconversion losses during storage in the battery and discharging from thebattery. On the other hand, the battery may have considerably smallerdimensions, which also makes the system according to the disclosurecost-effective in view of the fact that electrical storage technology isstill very expensive.

In another embodiment of the system according to the disclosure, thebidirectional DC chopper determines a currently required discharge powerfrom an input DC voltage applied to its first terminal according to thepredefined second characteristic curve. If the local energy consumptionis higher than the simultaneously available power from the local DCsource and if the applied input DC voltage falls below a predefinedthreshold, for example, this may be a signal to the bidirectional DCchopper to feed energy from the battery into the DC distribution panelof the system according to the disclosure. This makes it possible tofurther increase the self consumption of the locally generated energy.

If all connected loads are supplied from the local DC source and thestorage capacity of the battery is also fully used, further variableloads may be provided, for example, and are switched on in order toconsume excess local energy. The switching-on of the variable loadscould be triggered, for example, by means of a special signal (broadcastsignal) modulated onto the electrical lines. If further localconsumption is no longer possible, the local DC source can be curtailed,that is to say its power can be reduced such that local generation andconsumption are balanced out. A feed in of energy into the grid is notprovided in the system according to the disclosure.

A system according to the disclosure is distinguished by particularlycompact and cost-effective components and is ideally suited fordistributing the energy from a shared photovoltaic installation, forexample in an apartment building. This is also advantageous, inparticular, when retrofitting a building with a photovoltaicinstallation since only a few installation steps need to be carried outand only a few components need to be installed. Conventional electricaldistribution panels, in particular apartment distribution panels, have aconnection to the AC voltage distribution grid and are equipped with anelectricity meter which measures the energy consumption of theapartment/subunit or the like. The electrical power is distributed onlines which are equipped with fuses and are routed to the respectiveconsumption points.

One embodiment relates to a retrofit kit for an electrical distributionpanel, in particular an apartment distribution panel, for use in asystem according to the disclosure. In this case, the retrofit kitcomprises an inverter having an inverter output configured to beconnected to an alternating voltage input of the distribution panel. Theinverter has an inverter input configured to be connected to at leastone renewable DC source, and a controller which determines an input DCvoltage applied to the inverter input. The retrofit kit also comprises acommunication connection for transmitting power consumption data from ameter arranged in the distribution panel to the inverter, and aconnection for a DC distribution panel for connecting the distributionpanel to the renewable DC source. The controller determines a power tobe currently converted from the applied input DC voltage and transmittedpower consumption data.

In this manner, no further complicated installation work is requiredbeyond the installation of the renewable DC source, in particular aphotovoltaic installation on the roof of the building, and of a line asDC distribution panel through a central installation shaft in thebuilding and its routing to the apartment distribution panels. Since theinverter for a residential unit can be very compact in the case of anenergy consumption conventional for a household, the components of theretrofit kit can generally be accommodated in the existing apartmentdistribution panel. This makes it possible to subsequently install aphotovoltaic installation to be shared in a cost-effective andlow-complexity manner, in particular in an apartment building.

The disclosure is described below on the basis of example embodimentsusing drawings from which, in joint consideration with the features ofthe claims, further features, properties and advantages of thedisclosure emerge.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 shows a schematic illustration of an example configuration of asystem according to the disclosure;

FIG. 2 shows an example configuration of a load unit of a systemaccording to the disclosure; and

FIG. 3 shows an example configuration of a storage unit of a systemaccording to the disclosure.

DETAILED DESCRIPTION

FIG. 1 schematically shows an example configuration of a system 1according to the disclosure. Load units 3, 3′, 3″ are connected to agrid 10 at an grid connection 9. This may be, for example, a low-voltagedistribution grid of a public AC voltage grid which is generallyprovided by a grid operator and often has a multi-phase design. The loadunits 3, 3′, 3″ may be different apartments in an apartment building,floors or other subunits of a building or business. Furthermore, theload units 3, 3′, 3″ are connected to a DC distribution panel 12, towhich a shared DC source 2 is also connected. The DC source 2 may be,for example, a photovoltaic installation on the roof of an apartmentbuilding. In this case, a DC/DC converter (not shown) will usually alsobe arranged between the PV generator and the DC distribution panel 12,said controller controlling the maximum power point (MPP) of the PVgenerator.

FIG. 2 shows an example configuration of a load unit 3, as may beincluded in a system 1 according to the disclosure. The load unit 3 isconnected at its input 4 to the grid connection 9 and to a DCdistribution panel 12, as described above. Furthermore, the load unit 3also comprises an output 5, to which electrical loads 11, 11′, 11″, 11′″are connected via an AC distribution panel 16. The loads 11, 11′, 11″,11′″ may be conventional loads in a household such as light fittings,washing machines, cookers etc. but also production machines, for examplein a business. The loads 11, 11′, 11″, 11′″, . . . can be configured toconsume single-phase or multi-phase AC power and the AC distributionpanel 16 can distribute single-phase loads to a plurality of phases, asis conventional in a household, for example. The current powerconsumption from the grid 10 is determined in the load unit 3 by meansof a power meter 13. For this purpose, a power meter input 14 isconnected to the input 4. The power meter 13 may be an electricitymeter, as is conventionally present in an apartment in an apartmentbuilding, for example. However, this meter must be configured at leastto transmit the power consumption data currently measured by it.

The load unit 3 also comprises an inverter 6, the inverter input 7 ofwhich is connected to the DC distribution panel 12 and the inverteroutput 8 of which is connected to the connection between the input 4 andthe output 5 of the load unit 3. The inverter 6 is configured to convertDC power available at its input into grid-compliant AC power and to feedthe latter into the connection between the input 4 and the output 5 ofthe load unit 3 or the AC distribution panel 16.

The power meter 13 transmits its currently measured power consumptiondata to the inverter 6 via a power meter output 15. This transmissioncan take place in a wired or wireless manner. The idea on which thesystem 1 according to the disclosure is based is that of consuming thepower provided by the DC source 2 as completely as possible within thesystem 1. For this purpose, the inverter 6 may be configured in such amanner that it increases the power fed in by it until the powerconsumption from the grid has fallen to zero watts. All loads 11, 11′,11″, 11′″ connected to the load unit 3 are either then supplied from thelocal DC source 2 (or there is no consumption at all).

In order to determine whether the DC source 2 has power reserves, thatis to say whether the inverter 6 can further increase the power fed inby it in order to supply the connected loads 11, 11′, 11″, 11′″, . . .with more power, the inverter 6 can measure the DC voltage applied toits input 7, for example. The DC voltage may fall in the case of highloading of the DC source 2. The inverter 6 can conclude, for examplefrom a DC voltage which has fallen below a predefined threshold value,that it cannot increase the power converted by it any further. If the DCvoltage continues to fall, it is possible to store the fact that theinverter 6 reduces the power fed in by it. Conversely, a high DC voltagecan indicate that it is possible to increase the converted power.

Alternatively, the inverter 6 could determine a signal modulated ontothe input DC voltage as an indicator of the power reserves of the DCsource 2 at its inverter input 7. This signal can be modulated onto theinput DC voltage by a DC/DC converter (not shown) arranged between thePV generator and the DC distribution panel 12, and threshold values forthis signal can then be used to determine the power reserves of the DCsource 2, as described above. The threshold values or characteristiccurves for describing the relationship between the signal or DC voltagelevel and the power reserves of the DC source 2 can be stored in thecontroller (not shown) of the inverter 6.

The components of a load unit 3 can be in a conventional electricaldistribution panel of an apartment. Since the electricity meter and theconnections 4 and 5 are usually already present there, the distributionpanel can also be subsequently easily upgraded to form a load unit 3 foruse in the system 1 according to the disclosure by additionallyinstalling an inverter 6, routing the DC distribution panel 12 andsetting up a communication connection between the electricity meter 13and the inverter 6.

FIG. 3 shows an example configuration of a storage unit 20 for use in asystem 1 according to the disclosure. The storage unit 20 comprises abattery 22 and a bidirectional DC chopper (DC/DC converter) 21. Aterminal 23 of the DC/DC converter 21 is connected to the DCdistribution panel 12 and, via the latter, the DC/DC converter isconnected to the DC source 2. A terminal 24 of the DC/DC converter 21 isconnected to the battery 22, wherein the battery 22 may consist of aplurality of battery subunits or a plurality of batteries. The DC/DCconverter 21 acts, on the one hand, as a charging controller by takingenergy from the DC distribution panel 12 and storing it in the battery22 and, on the other hand, the DC/DC converter 21 also controls theremoval and feeding of energy from the battery 22 into the DCdistribution panel 12. In a similar manner to the load unit 3 describedabove, the DC/DC converter 21 can measure the DC voltage applied to itsterminal 23 or a signal modulated onto the input DC voltage as anindicator of the power reserves of the DC source 2. The controller ofthe DC/DC converter 21 stores a second characteristic curve whichestablishes a relationship between electrical characteristic variables,for example between the applied DC voltage level and an available DCpower or the maximum possible power which can be currently stored. Thissecond characteristic curve may have a different gradient than the firstcharacteristic curve, which results in a different response behavior ofload units and storage units with respect to the same measured DCvoltage level. This may mean that the loads connected to the load unitsare first of all supplied with electrical energy and the battery ischarged only secondarily.

If, for example, the measured DC voltage level falls below a further,lower threshold, this may indicate a higher local consumption ofelectrical power in comparison with the local generation. This may bethe signal for the storage unit to feed its stored energy into the DCdistribution panel 12 again. The previously locally excessive energy canthus be supplied for self consumption again.

The invention claimed is:
 1. A system for distributing locally generatedenergy from at least one renewable DC source to a plurality of localload units of the system, comprising, for each load unit: an inputterminal configured to connect to a grid, an output terminal configuredto connect to at least one load, an inverter comprising an inverterinput and an inverter output, wherein the inverter input is connected tothe at least one renewable DC source and the inverter output isconnected to the input terminal and to the output terminal of therespective load unit, and wherein the inverter is configured to converta direct current at the inverter input into an alternating current atthe inverter output, a power meter comprising a power meter inputconnected to the input terminal of the respective load unit, wherein thepower meter is configured to determine a current power consumption fromthe grid, and wherein the power meter comprises a power meter outputconnected to the inverter of the respective load unit, and wherein thepower meter is configured to transmit data relating to the current powerconsumption from the grid to the inverter, and wherein the inverter ofthe respective load unit is configured to determine an input DC voltageapplied to its inverter input and to determine a power to be currentlyconverted from the applied input DC voltage and the current powerconsumption data transmitted thereto.
 2. The system as claimed in claim1, wherein the inverter of the respective load unit is configured todetermine from the input DC voltage applied to its inverter input amaximum possible power which can be currently converted according to apredefined first characteristic curve.
 3. The system as claimed in claim2, wherein, if the maximum possible power which can be currentlyconverted is greater than or equal to the current power consumption fromthe grid, the inverter of the respective load unit is configured todetermine the power to be currently converted in such a manner that thecurrent power consumption from the grid reaches a preset limit value. 4.The system as claimed in claim 3, wherein the preset limit value is 0kW.
 5. The system as claimed in claim 2, wherein, if the maximumpossible power which can be currently converted is less than the powerconsumption from the grid, the inverter of the respective load unit isconfigured to determine the power to be currently converted in such amanner that it corresponds to a level of the maximum possible powerwhich can be currently converted, with the result that the current powerconsumption from the grid is minimized.
 6. The system as claimed inclaim 1, wherein the inverter is unidirectional and agalvanically-isolating inverter.
 7. The system as claimed in claim 2,wherein the first characteristic curves of the respective load unitsdiffer from one another.
 8. The system as claimed in claim 2, wherein,for at least one load unit, the power converted by the inverter of theload unit is summed up within a time window and a threshold value ispredefined for the summed power of the load unit, wherein the inverterof the load unit reduces its currently converted power to zero uponreaching the threshold value.
 9. The system as claimed in claim 2,further comprising a storage unit comprising at least one battery and abidirectional DC chopper, wherein a first terminal of the bidirectionalDC chopper is connected to the DC source and a second terminal of thebidirectional DC chopper is connected to the battery, and wherein thebidirectional DC chopper is configured to determine a maximum possiblepower which can be currently stored from an input DC voltage applied toits first terminal according to a predefined second characteristiccurve.
 10. The system as claimed in claim 9, wherein the predefinedsecond characteristic curve differs from the predefined firstcharacteristic curve.
 11. The system as claimed in claim 9, wherein thebidirectional DC chopper is configured to determine a currently requireddischarge power from an input DC voltage applied to its first terminalaccording to the predefined second characteristic curve.
 12. A retrofitkit for an electrical distribution panel, in particular an apartmentdistribution panel, for use in a system according to claim 1,comprising: an inverter, comprising: an inverter output terminalconfigured to connect to an alternating voltage input of distributionpanel, an inverter input terminal configured to connect to at least onerenewable DC source, and a controller configured to determine an inputDC voltage applied to the inverter input terminal and configured todetermine a power to be currently converted from the input DC voltageand a power consumption data, a communication connection configured totransmit the power consumption data from a meter arranged in thedistribution panel to the inverter, and a connection interfaceconfigured to connect the distribution panel to the renewable DC source.